Italy's Interior Minister Matteo Salvini has tried to ban the Dutch-flagged vessel from approaching under a "closed ports" policy, which has seen migrants repeatedly stranded at sea.

"We will use every lawful means to stop an outlaw ship, which puts dozens of migrants at risk for a dirty political game," Salvini said on Facebook after Sea-Watch said it was headed for Italy.

"I will not give permission for anyone to disembark... Our patience is over. The Netherlands will answer."

Salvini said Rome's ambassador to The Hague was "taking a formal step with the Netherlands government."

"Italy deserves respect: we expect the Netherlands to take in the immigrants on board," Salvini said.

Italian coastguard and customs vessels approached the Sea Watch as it headed for Lampedusa port.

The European Court of Human Rights (ECHR) in Strasbourg declined Tuesday to intervene but called on Italy to "continue to provide all necessary assistance" to vulnerable migrants.

The German NGO Sea-Watch had asked the ECHR to impose "interim measures" on Italy, saying the court could ask Rome to take urgent steps to resolve the standoff to "prevent serious and irremediable violations of human rights".

'Until Christmas'

Salvini said Tuesday the charity vessel could "stay there until Christmas and New Year" but would never be allowed in.

Of the 53 migrants initially rescued by the Sea-Watch 3 off Libya on June 12, Italy took in 11 vulnerable people.

Salvini earlier this month issued a decree that would bring fines of up to 50,000 euros ($57,000) for the captain, owner and operator of a vessel "entering Italian territorial waters without authorisation".

More than 400 migrants have arrived in Italy over the last two weeks aboard smaller vessels, while the Sea Watch has zig-zagged on the high seas without a safe port.

On Lampedusa, where Salvini's far-right League won 45 percent in May's European elections, a priest has camped in the street to demand those on board -- including three minors -- be allowed to disembark.

Dozens of German cities have said they are ready to welcome them, and the Bishop of Turin, Cesare Noviglia, said Monday his diocese would be willing to take them in.

"We can't hold on any longer. It's like we're in a prison because we are deprived of everything. Help us, think of us," one migrant from the Ivory Coast said in a video broadcast by Sea-Watch.

In January, 32 migrants rescued by the vessel were stranded on board for 18 days before they were allowed to disembark in Malta thanks to a distribution deal made between several European countries.

"It's serious when the captain has no other choice but to honour her sense of responsibility at the cost of personal consequences," said Carlotta Sami, spokeswoman for the UN refugee UNHCR in Italy.

"The UNHCR asks for the security decree to be revised and for a system of rescue and disembarkment to be created. The criminalisation of NGOs must end," she said.